Proteoglycans, major constituents of the extracellular matrix, are known to be present in large amounts in glial scar tissue and to inhibit recovery following spinal cord injuries (Fawcett & Asher, 1999). Enzymes that are capable of digesting glial scar tissue are an important target for the development of spinal cord injury (SCI) therapeutics. Chondroitinase ABCI (EC 4.2.2.4; cABCI) is a bacterial enzyme that catalyzes the digestion of sulfated chondroitin and dermatan side chains of proteoglycans. This enzyme has been shown to promote functional recovery after spinal cord injury (Bradbury et al., 2002; Caggiano et al., 2005).
The spinal cord is made up of nerve fibers. Damage to the central nervous system, including the spinal cord, results in a loss of function. Depending upon the type of injury to the central nervous system, the loss of function may manifest itself in loss of sensory, motor or autonomic function or a combination thereof. Sensory functions include the ability to feel sensations, like pain. Motor functions include the ability to voluntarily move your body. Autonomic functions include involuntary body functions, for example the ability to sweat and breathe.
The most common types of spinal cord injuries (SCI) include contusions (bruising of the spinal cord) and compression injuries (caused by prolonged pressure on the spinal cord). In contusion and compression injuries, a cavity or hole often forms in the center of the spinal cord. Unlike nerve cells, or neurons of the peripheral nervous system (PNS), neurons of the central nervous system (CNS) do not regenerate after injury.
Spinal cord injury can be characterized by contusion of the neural tissue with a resultant decrease or loss of the ability of nerve tissue to properly transmit nerve impulses. The usual cause is due to an impact injury of some nature, but it may also occur during the manipulation of the spinal cord in certain surgical procedures. After a spinal cord injury in the adult mammal, the inability of axons to regenerate may lead to loss of sensation, loss of motor function and/or loss of autonomic function, as well as permanent paralysis. One reason that neurons fail to regenerate is their inability to traverse the glial scar that develops following a spinal cord injury. The injury-induced lesion will develop glial scarring, which contains extracellular matrix molecules including chondroitin sulfate proteoglycans (CSPGs). CSPGs inhibit nerve tissue growth in vitro and nerve tissue regeneration at CSPGs rich regions in vivo.
A number of molecules, and specified regions thereof, have been implicated in the ability to support the sprouting of neurites from a neuronal cell, a process also referred to as neurite outgrowth. The term neurite refers to both axon and dendrite structures. The process of sprouting neurites is essential in neural development and regeneration, especially after physical injury or disease has damaged neuronal cells. Neurites elongate profusely during development both in the central and peripheral nervous systems of all animal species. This phenomenon pertains to both axons and dendrites.
Various polypeptides, especially cell adhesion molecules (CAMs), have been known to promote neural cell growth. While early efforts in this area of research concentrated on the adhesion-promoting extracellular matrix protein fibronectin (FN), other polypeptides have also been found to promote neural growth. For example, U.S. Pat. No. 5,792,743 discloses novel polypeptides and methods for promoting neural growth in the CNS of a mammal by administering a soluble neural CAM, a fragment thereof, or a Fc-fusion product thereof. U.S. Pat. No. 6,313,265 discloses synthetic polypeptides containing the pharmacologically active regions of CAMs that can be used in promoting nerve regeneration and repair in both peripheral nerve injuries as well as lesions in the CNS. While helpful, the use of regenerative proteins alone may not be sufficient to effect repair of a damaged nervous system.
During approximately the past two decades, knowledge of cell adhesion and migration in extracellular matrices (ECMs) at the molecular level has expanded rapidly. The action of enzymes and other polypeptides which degrade components of the extracellular matrix and basement membranes may facilitate the events of neural repair by a variety of mechanisms, including the release of bound cytokines and by increasing the permeability of the matrix, thereby enhancing the mobility of mediator molecules, growth factors and chemotactic agents, as well as the cells involved in the healing process. For example, U.S. Pat. No. 5,997,863 discloses the use of glycosaminoglycans to manipulate cell proliferation and promote wound healing.
Components of the inhibitory CSPGs have been identified as the glycosaminoglycans, chondroitin sulfate (CS) and dermatan sulfate (DS). Removal of these inhibitory molecules would allow neurites to regenerate and reinnervate an area after physical injury or disease, as well as to allow for the recovery of sensory, motor and autonomic functions.
Previous studies have found that chondroitinases can lyse and degrade CSPGs including, CS and DS. One study found that chondroitinase ABC removed glycosaminoglycan (GAG) chains in and around lesioned areas of rat CNS in vivo. The degradation of GAGs promoted expression of a growth-associated protein, GAP-43, indicating an increase in the ability of treated cells to regenerate. However, this growth-associated protein is associated with regeneration in peripheral, but not central, nerve injuries.
Chondroitin sulfates (CS) are sulfated polysaccharides in linear chains of a repeated dissacharides. They range in molecular weight from about 10,000 to over 100,000 Da. Chondroitin sulfate substrates exist in different isomers designated by the appended letters A, B, and C (Hoffman et al., 1958). The repeating units are composed of uronic acid (GlcA or IdoA) and galactosamine, and are called galactosaminoglycans, and are one example of the glycosaminoglycans, typically abbreviated as GAG. Although these GAG chain species have different repeating disaccharide regions, they are covalently bound through the so-called linkage region tetrasaccharide sequence (see below) to the serine residue in the GAG attachment consensus sequence (Glu/Asp-X-Ser-Gly) of respective core proteins. Chondroitin A and C sulfates (ChS-A, ChS-C) are the most abundant GAGs and are found in cartilage, bone and heart valves. Chondroitin B (ChS-B, or, alternatively, dermatan sulfate) is expressed mostly in skin, blood vessels, and heart valves.
When chondroitinase bacterial preparations were characterized against different chondroitin sulfate (ChS) substrates, a series of distinct chondroitinases were discovered: Chondroitinase AC that degrades mostly chondroitin A (ChA) and chondroitin C (ChC) (Yamagata et al., 1968), Chondroitinase B that degrades chondroitin B (ChB) (Michelacci and Deitrich, 1976), Chondroitinase C that acts mostly on ChC (Michelacci Y M & Dietrich C P, 1976) and Chondroitinase ABC exhibits specificity against all three substrates—ChS-A, ChS-B and ChS-C (Yamagata et al., 1968, Michelacci et al., 1987).